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Tidal marsh plant species commonly zonate along environmental gradients such as elevation, but it is not always clear to what extent plant distribution is driven by abiotic factors vs. biotic interactions. Yet, the distinction has importance for how plant communities will respond to future change such as higher sea level, particularly given the distinct flooding tolerances and contributions to elevation gain of different species. We used observations from a 33-year experiment to determine co-occurrence patterns for the sedge, Schoenoplectus americanus, and two C4 grasses, Spartina patens and Distichlis spicata, to infer functional group interactions. Then, we conducted a functional group removal experiment to directly assess the interaction between sedge and grasses throughout the range in which they cooccur. The observational record suggested negative interactions between sedge and grasses across sedge- and grass-dominated plots, though the relationship weakened in years with greater flooding stress. The removal experiment revealed mutual release effects, indicating competition was the predominant interaction, and here, too, competition tended to weaken, though nonsignificantly, in more flooded, lower elevation zones. Whereas zonation patterns in undisturbed portions of marsh suggest that the sedge will dominate this marsh as flooding stress increases with sea level rise, we propose that grasses may exhibit a competition release effect and contribute to biomass and elevation gain even in sedge-dominated communities as sea level continues to rise. Even as abiotic stresses drive changes in the relative contributions of sedges and grasses, competition among them moderates fluctuations in total plant biomass production through time. 

Thin layer sediment placement (TLP) is used to build elevation in marshes, counteracting effects of subsidence and sea level rise. However, TLP success may vary due to plant stress associated with reductions in nutrient availability and hydrologic flushing or through the creation of acid sulfate soils. This study examined the influence of sediment grain size and soil amendments on plant growth, soil and porewater characteristics, and greenhouse gas exchange for three key U.S. salt marsh plants:Spartina alterniflora(synonymSporobolus alterniflorus),Spartina patens(synonymSporobolus pumilus), andSalicornia pacifica.We found that bioavailable nitrogen concentrations (measured as extractable NH₄⁺‐N) and porewater pH and salinity were inversely related to grain size, while soil redox was more reducing in finer sediments. This suggests that utilizing finer sediments in TLP projects will result in a more reduced environment with higher nutrient availability, while larger grain sized sediments will be better flushed and oxygenated. We further found that grain size had a significant effect on vegetation biomass allocation and rates of gas exchange, although these effects were species‐specific. We found that soil amendments (biochar and compost) did not subsidize plant growth but were associated with increases in soil respiration and methane emissions. Biochar amendments were additionally ineffective in ameliorating acid sulfate conditions. This study uncovers complex interactions between sediment type and vegetation, emphasizing the limitations of soil amendments. The findings aid restoration project managers in making informed decisions regarding sediment type, target vegetation, and soil amendments for successful TLP projects. 

Abstract Stocks and fluxes of soil inorganic carbon have long been ignored in the context of coastal carbon sequestration, and their implications for the climate cooling effect of blue carbon ecosystems are complex. Here, we investigate the role of soil inorganic carbon in five salt marshes along the northern coast of the European Wadden Sea, one of the world's largest intertidal areas, harboring ~ 20% of European salt‐marsh area. We demonstrate a substantial contribution of inorganic carbon (average: 29%; range: 7–57%) to the total soil carbon stock of the top 1 m. Notably, inorganic exceeded organic carbon stocks in one of the studied sites; a finding that we ascribe to site geomorphic features, such as proximity to marine calcium carbonate sources and hydrodynamic exposure. Contrary to our hypothesis that inorganic carbon stocks would decline along the successional gradient from tidal flat to high marsh, as carbonate deposits would progressively dissolve in increasingly organic‐rich rooted sediments, our findings demonstrate the opposite pattern—an increase in inorganic carbon stocks along the successional gradient. This suggests that the dissolution of calcium carbonates in the root zone is counterbalanced by other processes, such as trapping of sedimentary carbonates by marsh vegetation and calcium carbonate precipitation in anaerobic subsoils. In the context of blue carbon, it will be critical to develop an improved understanding of these plant‐ and microbiota‐mediated processes in calcium carbonate cycling.